Open RTOS or Free RTOS
Formal
FreeRTOS is a popular real-time operating system kernel for embedded devices, that has been ported to 35 microcontrollers.
Practical
It is distributed under the GPL with an optional exception. The exception permits users' proprietary code to remain closed source while maintaining the kernel itself as open source, thereby facilitating the use of FreeRTOS in proprietary applications. OPENRTOS provides the only available commercial license for FreeRTOS.
